The present invention relates to carbon foam hybrid/composite electrodes, particularly to thin film carbon hybrid/composite electrodes, and more particularly to a method for forming thin, flat porous carbon sheets by infiltrating a highly porous fiber matrix with a carbon foam precursor material, curing to form a polymer-saturated material, and pyrolyzing to form a thin sheet of carbon, for use as high density and/or high energy density power electrodes.
Carbon aerogels or foams produced by pyrolyzing organic precursors are known in the art, as exemplified by U.S. Pat. No. 4,873,218 issued Oct. 10, 1989; U.S. Pat. No. 4,997,804 issued Mar. 5, 1991; and U.S. Pat. No. 5,086,085 issued Feb. 4, 1992, each to R. W. Pekala. Also, recent efforts have been directed to the use of monolithic carbon aerogel electrodes for use in capacitors, etc., as exemplified by U.S. application Ser. No. 07/822,438 filed Jan. 17, 1992, entitled "Supercapacitors Based On Carbon Foams", in the name of J. L. Kaschmitter et al. now U.S. Pat. No. 5,260,855 issued Nov. 9, 1993; and U.S. application Ser. No. 08/090,881, filed Jul. 8, 1993 entitled "Cell Separator For Use In Bipolar-Stack Energy Storage Devices", in the name of S. T. Mayer et al.
Thin films of porous carbon are desired for use as electrodes in batteries and capacitors because they exhibit lower resistance and can provide high efficiency and high power capability. Thinner films also provide short electrolyte diffusion distances and better access to the carbon. Thin layers or films (&lt;20 mils) of the prior monolithic carbon foams can be difficult to form and handle because they often are brittle. Processing of monolithic thin films is difficult due to the large amount of shrinkage that accompanies the curing and pyrolyzing, sometimes resulting in cracking of the material as it shrinks. Thus, there is a need for a method to form thin carbon foam hybrid/composite electrodes which overcome the disadvantages of the prior known monolithic carbon foams for use as electrodes.
The present invention satisfies this need by providing a method for producing thin, flat porous carbon electrodes with an electrically continuous, low resistance structure. The method involves the infiltration of a highly porous, low density (&lt;0.2 g/cc) material, such as carbon paper or felt, with the appropriate carbon foam precursor material prior to pyrolysis. The infiltrated highly porous material (referred to hereinafter as the "substrate") allows for the carbon foam to shrink around fibers of the material, rather than around itself in the formation of a monolithic structure, thereby eliminating the above-mentioned shrinkage problem of the prior known monolithic carbon foams. The use of the porous substrate material can also allow for roll-to-roll processing of thin sheets of the carbon foam for improved manufacturability.